Resistance spot welding is a process used by a number of industries to join together two or more metal workpieces. The automotive industry, for example, often uses resistance spot welding to join together pre-fabricated metal workpieces during the manufacture of a vehicle door, hood, trunk lid, or lift gate, among others. A number of spot welds are typically formed along a peripheral region of the metal workpieces or some other bonding region to ensure the part is structurally sound. While spot welding has typically been practiced to join together certain similarly-composed metal workpieces—such as steel-to-steel and aluminum alloy-to-aluminum alloy—the desire to incorporate lighter weight materials into a vehicle platform has generated interest in joining steel workpieces to aluminum alloy workpieces by resistance spot welding. Moreover, the ability to resistance spot weld workpiece stack-ups containing different workpiece combinations (e.g., aluminum alloy/aluminum alloy, steel/steel, and aluminum alloy/steel) with one piece of equipment would increase production flexibility and reduce manufacturing costs.
Resistance spot welding, in general, relies on the resistance to the flow of an electrical current through overlapping metal workpieces and across their faying interface to generate heat. To carry out such a welding process, a pair of opposed spot welding electrodes are typically clamped at diametrically aligned spots on opposite sides of the workpieces at a predetermined weld site. An electrical current is then passed through the metal workpieces from one electrode to the other. Resistance to the flow of this electrical current generates heat within the metal workpieces and at their faying interface. When the metal workpieces being spot welded together are a steel workpiece and an aluminum alloy workpiece, the heat generated at the faying interface initiates a molten weld pool extending into the aluminum alloy workpiece from the faying interface. The molten aluminum alloy weld pool wets the adjacent surface of the steel workpiece and, upon cessation of the current flow, solidifies into a weld nugget that forms all or part of a weld joint between the two workpieces.
In practice, however, spot welding a steel workpiece to an aluminum alloy workpiece is challenging since a number of characteristics of those two metals can adversely affect the strength—most notably the peel strength—of the weld joint. For one, the aluminum alloy workpiece usually contains one or more refractory oxide layers (hereafter collectively “oxide layer”) on its surface. The oxide layer, which is composed primarily of aluminum oxides but may also include other oxides, such as magnesium oxides, is electrically insulating and mechanically tough. The surface oxide layer thus raises the electrical contact resistance of an aluminum alloy workpiece—namely, at its faying surface and at its electrode contact point—making it difficult to effectively control and concentrate heat within the aluminum alloy workpiece, and has a tendency to hinder the ability of the molten weld pool to wet the steel workpiece. And while efforts have been made in the past to try and remove the oxide layer from the aluminum alloy workpiece prior to spot welding, such practices can be impractical since the oxide layer has the ability to regenerate in the presence of oxygen, especially with the application of heat from spot welding applications.
The steel workpiece and the aluminum alloy workpiece also possess different properties that tend to complicate the spot welding process. Specifically, steel has a relatively high melting point (˜1500° C.) and relatively high thermal and electrical resistivities, while aluminum alloy has a relatively low melting point (˜600° C.) and relatively low thermal and electrical resistivities. As a result of these physical differences, most of the heat is generated in the steel workpiece during electrical current flow. This heat imbalance sets up a temperature gradient between the steel workpiece (higher temperature) and the aluminum alloy workpiece (lower temperature) that initiates rapid melting of the aluminum alloy workpiece. The combination of the temperature gradient created during current flow and the high thermal conductivity of the aluminum alloy workpiece means that, immediately after the electrical current has ceased, a situation occurs where heat is not disseminated symmetrically from the weld site. Instead, heat is conducted from the hotter steel workpiece through the aluminum alloy workpiece towards the welding electrode in contact with the aluminum alloy workpiece, which creates steep thermal gradients in that direction.
The development of steep thermal gradients between the steel workpiece and the welding electrode in contact with the aluminum alloy workpiece is believed to weaken the integrity of the resultant weld joint in two primary ways. First, because the steel workpiece retains heat for a longer duration than the aluminum alloy workpiece after the electrical current has ceased, the molten weld pool that has been initiated and grown in the aluminum alloy workpiece solidifies directionally, starting from the region nearest the colder welding electrode (often water cooled) associated with the aluminum alloy workpiece and propagating towards the faying interface. A solidification front of this kind tends to sweep or drive defects—such as gas porosity, shrinkage voids, micro-cracking, and oxide residue—towards and along the faying interface within the aluminum alloy weld nugget. Second, a sustained elevated temperature in the steel workpiece promotes the growth of brittle Fe—Al intermetallic compounds at and along the faying interface. The intermetallic compounds tend to form thin reaction layers between the aluminum alloy weld nugget and the steel workpiece. These intermetallic layers are generally considered part of the weld joint, if present, in addition to the weld nugget. Having a dispersion of weld nugget defects together with excessive growth of Fe—Al intermetallic compounds along the faying interface is thought to reduce the peel strength of the final weld joint.
In light of the aforementioned challenges, previous efforts to spot weld a steel workpiece and an aluminum alloy workpiece have employed a weld schedule that specifies higher currents, longer weld times, or both (as compared to spot welding steel-to-steel), in order to try and obtain a reasonable weld bond area. Such efforts have been largely unsuccessful in a manufacturing setting and have a tendency to damage the welding electrodes. Given that previous spot welding efforts have not been particularly successful, mechanical processes such as self-piercing rivets and flow-drill screws have been used predominantly instead. Both self-piercing rivets and flow-drill screws are considerably slower and have high consumable costs as compared to spot welding. They also add weight to the vehicle body structure, which at some point can begin to counteract the weight savings attained through the use of aluminum alloy workpieces in the first place. Advancements in spot welding that would make the process more capable of joining steel and aluminum alloy workpieces would thus be a welcome addition to the art.